In general, in optical transmission and reception devices (and optical transmission and reception methods), there are applied, as optical modulation methods, On Off Keying (OOK), Binary Phase Shift Keying (BPSK), Differential Phase Shift Keying (Differential PSK: DPSK), Quadrature Phase Shift Keying (QPSK), Differential Quadrature Phase Shift Keying (Differential QPSK: DQPSK), and so on.
In multi-level modulation optical transmission and reception devices to which the above-mentioned optical modulation methods are applied, a technique is used which multiplexes a transmission signal by the use of phase information or a polarized wave state (polarization state), instead of binary values of “1, 0”.
At this time, a signal is transmitted in a communication channel using a plurality of logic lanes, but there is a problem in which the signal can not be correctly decoded at a receiver side due to the facts that a difference in transmission time among the lanes, called a skew, may occur during transmission from a transmitter to a receiver, and a swapping or exchange of the lanes may be caused by the input state of polarized waves at a reception front end part.
Accordingly, in the past, there has been proposed a technique called OTN-MLD (Optical Transport Network Multi-Lane Distribution) for regulating such a skew (for example, see a first nonpatent document).
Although the OTN-MLD has a function to correct the lanes which were swapped or exchanged with each other in the reception front end part besides skew correction (deskew), it is necessary to carry out a barrel shift (lane rotation) of a frame alignment signal (FAS) in units of OTUk (k=0, 1, 2, . . . ) (Optical Transport Unit) frame at a transmitter side, in order to carry out skew correction as well as lane exchange or correction in the MLD.
This is because in a state before the barrel shift is carried out, an FAS exists only in one certain lane, but in order to measure the skew states of the plurality of lanes at the receiver side, frame alignment signals (FASs) have to exist in all the lanes, respectively, and at the same time, it is necessary to observe the skew states based on changes in the relative positions of the frame alignment signals.
Here, as the signal processing operation of the above-mentioned conventional device, reference will be made to an operation thereof in cases where differential decoding is carried out by means of a reception front end part, a multi-lane synchronization (OTN-MLD) part and a differential decoder part, while taking, as an example, a receiver of a digital signal processing optical transceiver.
First, in the reception front end part, an optical reception signal is separated into an I channel and a Q channel of an X polarized wave component, and is also separated into an I channel and a Q channel of a Y polarized wave component, so that it is thus separated into a total of four channels (four lanes).
In the multi-lane synchronization part, OTUk frames received therein are subjected to “skew correction (deskew)” processing to correct skews between the lanes which are generated in the communication channel and the reception front end part, and at the same time, they are also subjected to “lane exchange” processing to restore the change of lanes generated in the communication channel, the reception front end part and an adaptive equalization filter part, and as well as to “lane rotation” processing to restore the frames which have been barrel shifted for each OTUk frame in the multi-lane distribution part.
After that, the OTUk frames thus processed are inputted to the differential decoder part, but they have already been barrel shifted in units of OTUk frame in the multi-lane synchronization part, so an OTUk frame, which was an I channel of X polarization in its state of “0”, becomes a Q channel of X polarization in its state of “1”.
Accordingly, on the boundaries of the OTUk frames, signals, of which lanes are different from each other before and after each boundary, will be differentially decoded, and hence, there will be a possibility that the differentially encoded signal may not be decoded in a correct manner.